The present invention relates to a method for using an apparatus for profiling and dimensionalizing an interior cross-sectional portion of a tubular structure and the apparatus itself and, more particularly, to a noncontact method for profiling a tubular structure in off-line-and on-line (or "in-line") processes.
Present methods of measuring inside dimensions of tubular structures employ bore gauges, inside calipers, stylus profilometers, and ultrasonic techniques.
Bore gauges, stylus profilometers, and inside calipers all work on the principal of one or more tips contacting the inside surface of a tubular structure such that dimensions are determined by the amount of calibrated extension or translation the tips make relative to a reference position. Bore gauges typically have two or three tips or fingers 180.degree. or 120.degree. apart respectively, which contact the inner surfaces and are calibrated to read out inside diameters. While bore gauges are very precise, they are best suited for measuring rigid cylindrical structures and not well suited for measuring complex interior shapes or for measuring structures with soft or elastic inner surfaces such as rubber.
A modification of a bore gauge is a set of plugs of various diameters, which are inserted into a tubular structure until the closest match is found. These gauges can only measure the minor diameter of a tubular structure, that is, the smallest diameter of a tubular structure which does not have a circular cross section. In addition, a wide range of plug sizes is required for high precision work. The use of plugs requires the work piece to be in a stationary state.
Calipers work in a similar manner as bore gauges but usually are not designed to extend more than a few inches into a tubular structure and are also not well suited for measuring noncircular cross sections or soft materials. Furthermore, use of calipers relies on human feel, which contributes to inaccuracy in measurements and lack of repeatability in measurements.
Stylus profilometers trace out dimensions by translating a stylus tip over the surface and can be used inside tubular structures by extending the tip on long extension arms. The drawback to this method is that stylus profilometers are better suited for measurements along the axis of a tubular structure rather than measurements of cross-sectional profiles. Either the part or the profilometer would have to be rotated about the axis of the tubular structure, which poses serious mechanical and registration problems. Profilometers can determine the shape and relative dimensions of the interior of tubular structures, but absolute diameters are more difficult.
In addition to the above limitations, all of the devices discussed so far make contact with the surface being measured. Bore gauges, plug gauges and calipers cannot be used to measure moving parts. Furthermore, stylus profilometers are too slow to make cross-sectional dimensional measurements on moving parts. If the part to be measured is moving, such as during extrusion or tube forming, contact methods are inappropriate because they may disturb the process, they may come in contact with material that is viscous or not completely set, or the measurement may be biased by the motion of the moving part.
Ultrasonic detection is a noncontact technique which works by measuring the time of flight of a sound pulse from a transmitter to a receiver when the sound bounces off boundaries between materials of different density. In the case of tubular structures with more or less constant wall thickness, ultrasonic detectors can detect reflections off the exterior and interior walls of the tubular structure thereby measuring wall thickness via time of flight measurements of the sound pulse. Wall thickness combined with outside measurements indirectly determines inside diameter. However, ultrasonic techniques have low spatial resolution such that a complex interior structure would tend to appear as an average wall thickness, so ultrasonic techniques are best suited for nominally cylindrical structures. Low density materials such as rubber and plastic can be difficult to measure with ultrasonics since the magnitude of reflected sound is small when the density difference at a boundary is small. Tubular structures made up of layered materials can also pose problems. In general, ultrasonic measurements are carried out under water or other suitable liquid medium, which further limits the application possibilities. Ultrasonic techniques can also be too expensive for many applications.
Another device that could potentially be employed, if interior dimensions are large enough, is a standard laser displacement sensor head which measures displacement by triangulation, but these devices are typically several inches long and require a certain standoff distance which limits their use inside tubular structures to minimum clearances of about four to five inches in diameter.
Current state-of-the-art laser triangulation sensors measure displacements of a surface toward and away from the sensor head by emitting a laser beam which illuminates a spot on the surface. An optical imaging subsystem views the laser spot along a line of sight that is not parallel to the laser beam. The beam spot is imaged onto a position sensitive detector (PSD). As the distance between the surface and sensor changes along the laser beam direction, the imaged laser spot moves laterally along the PSD. The electrical output of the PSD is proportional to the position of the imaged laser spot on its light sensitive area which in turn is proportional to the distance between the sensor and the surface.
A more recent attempt at determining physical characteristics of an inside surface of a bore having an extremely small diameter is disclosed in U.S. Pat. No. 5,004,339 to Pryor et al. Pryor et al. discloses an apparatus that utilizes a guided wave parabolic index fiber to permit the transportation of light into a bore, such as a human throat or vein, for determining the physical characteristics of the throat or vein. This apparatus is similar to an endoscope. The light is focused directly by a spherical mirror onto the interior surface of the throat or vein. As a result of the focusing of the light, the range of interior diameters of bores that can be measured is inherently limited. In addition, this reference does not teach that the apparatus can be used in an on-line process, in other words, the bore is stationary.
Accordingly, there is a need for a manufacturing line tool for direct, noncontact measurement of interior cross-sectional dimensions in tubular structures, especially for complex interior surfaces like helical pump stators and gun barrel rifling; a noncontact measuring device for off-line and on-line process control, especially for soft materials and moving parts; a miniaturized device for use with small bores and tubular structures having small inside diameters; and a measuring tool that reduces scrap and provides quick, accurate and repeatable measurements.